J. Org. Chem. 1982,47,4129-4134
4129
New Bicyclic Diterpenoids from the Caribbean Gorgonian Octocoral Eunicea calyculata Sally A. Look and William Fenical* Institute of Marine Resources, Scripps Institution of Oceanography, La Jollo, California 92093
Received May 3, 1982 Two new bicyclic diterpenoids, 1 and 2, of the dolabellane ring system have been isolated from the Caribbean sea whip, Eunicea calyculata. The structures of these new compounds were assigned on the basis of chemical and spectral studies and particularly proton difference decoupling (DDS) and nuclear Overhauser enhancement difference spectroscopy (NOEDS).The facile BFa-inducedcyclization of one of these bicyclic diterpenoids to yield tricyclic compounds of the clavularane (dolashe) ring system illustrates the chemical relationship between these two diterpenoid classes. Marine octocorals of the order Gorgonacea (Cnidaria), commonly referred to as the sea whips and sea fans, are recognized as a rich source of biologically active and structurally unique secondary metabolites.' In the Caribbean Sea, sea whips of the genus Eunicea (family Plexauridae) are particularly abundant, and several chemical investigations of Eunicea species have been reported. Two classes of diterpenoid molecules have been isolated. Cembrane derivatives are by far the most common; in total, some 7 cembranoids have been reported2 from 6 of the 15 varieties of Eunicea found in the Caribbean Sea.3 Fuscol, a diterpenoid structurally similar to the elemane sesquiterpenoids,4 represents the sole noncembranoid diterpene isolated from this genus. In this paper we report the structures of two new bicyclic diterpenoids, 1 and 2, which were isolated from the Car-
2
1
16
li
3
4
ibbean gorgonian E. calyculata (Ellis and Solander). These bicyclic metabolites are new examples of diterpenoids of the dolabellane ring system: which was first isolated from the opisthobranch mollusc Dolabella calif ~ r n i c a . ~Later, .~ these compounds were recognized as ~~~~~
~
(I) Turech, B.; Braekman, J. C.; Daloze, D.; Kaisin, M. In 'Marine Natural Products Chemistry, Chemical and Biological Perspectives"; Scheuer, P. J., Ed.;Academic Press: New York, 1978; Vol. II,pp 247-296. (2) Hossain, M. B.; Nicholas, A. F.; van der Helm, D. J. Chem. SOC., Chem. Commun. 1968,386. van der Helm, D.; Enwall, E. L.; Weinheher, A. J.; Karns, T. K. B.; Ciereazko, L. 5.Acto Crystollogr., Sect. B. 1976, 1558. Chang, C. Y.; Ciereazko, L. S.; Hoseain, M. B.; van der Helm, D. Ibid. 1980,731. Ealick, S. E.; van der Helm, D.; Weinheimer, A. J. Ibid. 1975,1618. Weinheimer, A. J.; Matson, J. R.; van der Helm, D.; Poling, M. Tetrahedron Lett. 1977,1295. Groes, R. A. Ph.D. Thesis, University of Oklahoma, Norman, OK, 1974. (3) Eayer, F. M. "The Shallow-Water Odocorallia of the West Indian Region"; Nijhoffi The Hague, 1961. (4) Gopichand, Y.; Schmitz, F. J. Tetrahedron Lett. 1978, 39, 3641. (5) Ireland, C.; Faulkner, D. J.; Finer, J.; Clardy, J. J. Am. Chem. Soc. 1976,98,4664.
metabolites of brown seaweeds of the family Dictyotaceae.'p8 The isolation of dolabellane diterpenoids from E. calyculata represents an extension of the biosynthetic capability of sea whips of the genus Eunicea, and of cnidarians in general. E. calyculata was collected in deeper waters (-30 m) along the barrier reef of Belize and in the Bahama Islands. Freshly collected animals were either stored in 2-propanol or frozen and subsequently extracted with chloroform/ methanol (2:l). Diterpenoids 1 and 2 were purified by standard silica gel chromatography of the crude extract, followed by high-performance liquid chromatography (HPLC) of several of the relatively nonpolar fractions. Among the complex mixtures of secondary metabolites observed? the diterpenoids, 1 and 2, were by far the major components, comprising 9 and 12% of the organic extract, respectively. The epoxide 1 crystallized from diethyl ether after HPLC purification and analyzed for C20H3002by highresolution mass and % NMR spectrometry. The two oxygen atoms in 1 were readily assigned to a trisubstituted epoxide and an a,@-unsaturatedketone constellation by evaluation of spectral information. I3C NMFl bands (Table I) at 6 60.4 (s) and 65.7 (d), in conjunction with 'H NMR resonances a t 6 2.91 (1 H, br d, J = 8.5) and 6 1.36 (3 H, s), illustrated 1 to possess a methyl-substituted epoxide group. 13C NMR bands a t 6 206.4 (s), 149.0 (s), and 135.9 (8) [or 137.6 (s)], infrared absorptions a t 1700 and 1615 cm-', and U V absorption at 254 nm (e 9200) indicated that 1 possessed a fully substituted a,@-unsaturatedketone chromophore. Further consideration of the NMR characteristics of 1 showed the molecule to possess an additional nonconjugated olefin and three olefinic methyl groups, two of which were unusually deshielded [6 2.26 (3 H, s) and 1.93 (3 H, s)]. These data required that the fully substituted a,@-unsaturatedketone bear two methyl groups in one of two possible configurations: a vicinally disubstituted endocyclic olefin as in a, or a geminally disub-
;se.,
q
c
(a 1
H
CH3
CH 3 (b)
3 H3C ( C )
stituted exocyclic olefin as in b. Comparison of the spectral (6) Ireland, C.; Faulkner, D. J.; J. Org. Chem. 1977, 42, 3157. (7) Sun, H. H.; Fenical, W. Phytochemistry 1979,18, 340. (8) Amico, V.; Oriente, G.; Piattelli, M.; Tringali, C.; Fattorusso, E.; Magno, S.; Mayol, L. Tetrahedron 1980,36, 1409. (9) A complex mixture of monocyclic metabolites, some of the cembrane class, were also isolated.
0022-3263/82/1947-4129$01.25/00 1982 American Chemical Society
4130 J . Org. Chem., Vol. 47, No. 21, 1982
data with numerous model systemsloclearly indicated that 1 possessed the a-isopropylidenyl ketone functionality as in b. Furthermore, the chemical shift for the carbonyl carbon in the 13C NMR spectrum of 1 (206.4 ppm), the infrared absorption a t 1700 cm-l, and the UV absorption data were virtually identical with those data reported for 2-isopropylidenylcyclopentanone,"thus allowing the further assignment of this chromophore in a five-membered ring. Proton NMR data yielded even more information that defined the substitution pattern of the a-isopropylidenylcyclopentanone group in 1. One bridgehead methyl group [6 1.19 (3 H, s)], one allylic methine proton at 6 2.69 (1 H, br d, J = 12.6 Hz), and an AB double doublet a t 6 2.38 and 2.12 (J = 18.6 'Hz) were observed in the proton NMR spectrum of 1. These features could only be accommodated by the cyclopentanone constellation as fully defined in c. In order to confirm this assignment, epoxide 1 was treated with lithium in ammonia to selectively reduce the conjugated chromophore. Two non-UVabsorbing products, 5 and 6, were obtained; each com1
Li / N H ,
'
,:&
Look and Fenical
a ) DDS RESULT, IRRADIATION OF O L E F I N I C PROTON
b ) COMPUTER S I M U L A T I O N
I 25
I
I
20
15
IO
PPM
d 5
R = H , R'=cH(cH,),
6
R=CH(CH,),, R'=H
pound was purified by HPLC and fully characterized. Both products showed simple cyclopentanone carbonyl infrared absorptions a t 1735 cm-', and two new doublet methyl groups were present at high field in their 'H NMR spectra. Irradiation of a complex one-proton band at high field in each spectrum caused the doublet methyl groups to collapse to singlet resonances, thereby proving their assignments in isopropyl groups. Mass spectral and 13C NMR data showed 1 to possess two carbocyclic rings. Since 1 could be defined as possessing a five-membered ring with no other substituents except for methyls and the epoxide in the second ring, the remaining ring had to be 11-membered. The dolabellanes are a known class of diterpenoids possessing a bicyclo[9.3.0] ring system as in 3. However, an alternative structure, 4, which is also regularly terpenoid, or other structures with irregular methyl substitution patterns in the 11-membered ring could not be excluded as possibilities for the carbon skeleton of 1. In order to position the trisubstituted olefin at C3-C4 with an olefinic methyl at C4, a proton NMR experiment involving difference decoupling spectroscopy12 (DDS) was performed. While the trisubstituted olefin could be placed in numerous positions within the 11membered ring, only in the C3-C4 position, as in 3, would this olefin be proximate to a methylene group (C2), which would be isolated by the quaternary center at C1. Figure 1 illustrates the results of difference decoupling of the lone olefin proton in 1, which appears at 6 5.43 (dd, J = 11.6 (10)The exceptionally deshielded syn-methyl groups in a-isopropylidenyl ketones are highly characteristic of this chromophore. For (+)-pulegone, see: Pouchert, C. J.; Campbell, J. R. 'The Aldrich Library of NMR Spectra"; Aldrich Chemical Co., Inc.: Milwaukee, WI, 1974;Vol. 11, pp 140. (11)2-Isopropylidenylc clopentanone shows infrared absorption at 1706 ( v c - 0 ) and 1633 cm-Y ( Y C ~ O ) [see Erskine, R. L.; Wai ht, E. S. J. Chem. SOC.1960,34251,ultraviolet absorption at 252 nm (logEl t 3.56)[see French, H. S.; Wiley, L. J.Am. Chem. Soc 1949,71,37021,and I3C NMR bands at 204.6,130.2,and 144.5 ppm, respectively, for the carbonyl, and a and p exocyclic carbon atoms [see Marr, D. H.; Stothers, J. B. Can. J . Chem. 1965,43,5961. (12) Hall, L. D.; Sanders, J. K. M. J. Am. Chem. SOC.1980,102,5703.
Figure 1. Results of a proton difference decoupling experiment (DDS) with epoxide 1 at 360 MHz in CDC13 solution. (a) The result of spectra subtraction after irradiation at 6 5.43. (Methyl resonances were incompletely nulled.) (b) The results of a computer simulation (using a Nicolet 1180E computer program) of the difference experiment with JaC = 11.6, Jab= 4.7, and Jbc = 11.8 Hz. (c) Normal spectrum showing the region between 6 1.0 and 2.5.
H
(d) proton ( S ) irradiated
b
d
Trotanls) enhanced
-1 Estimated i n t e r i u c l e a r distanceslA)
b,d,l
1.6, 2 . 4 , 2 . 4
a,d
1.6, 2 . 2
e,f
2.2, 2.2
a,b
2.4, 2 . 2 2.4
C,f
2.2,
f
c,e.a
2.2, 2.4, 2.2
q
f
2.2
Figure 2. Results of a nuclear Overhauser enhancement difference spectroscopy (NOEDS) experiment with epoxide 1. Estimated internuclear distances were obtained with Dreiding models. The proton labeled "jnrepresenta the a proton only, which is within nOe proximity. and 4.7 Hz). The methylene protons a t C2 were very clearly defined by this experiment a t 6 2.16 and 1.63 by the difference pattern illustrated in Figure la. The geminal coupling constant was measured as 11.8 Hz, and further couplings of 11.6 and 4.7 Hz to the olefin proton were measured, since they could be eliminated in the decoupled component. Using the observed coupling constants, a computer simulation (Figure l b ) yielded superimposable results. Based upon these data, epoxide 1 was proposed to possess the dolabellane skeleton as in 3, void, however, of stereochemical refinements. In order to determine the relative spatial arrangements of all substituents and, at the same time, the conformation
New Bicyclic Diterpenoids
J. Org. Chem., Vol. 47, No. 21, 1982
4131
Table I. 'H and 13CNMR Assignments for the Eunicea calyculata Metabolites and the Two Key Derivativesasb epoxide 1 triene 2 eX0-7 endo-8 C no. 'H 13C 'H '3C 'H l3C 'H '3C 1 41.0 40.9 39.6 40.1 2.a 1.63 (1H, dd, 1.60 (1H. dd. J = i l . 7 , 5.0) J = 11.8, 4.7) 40.1e C 42.3 C 41.1 39*8e 2.13 (1H, dd, 20 2.16 (1H, dd, J = 11.6, 11.8) J = 11.7. 11.2) 124.7 5.24 (1H,'dd, ' 124.8 3 5.43 (1H, dd, 47.3 C 47.0 C J = 11.6, 4.7) J = 5.0, 11.2) 4 137.6d 149.4d 135.5d 137.gd 2.41 (1H, m) 5a 38.1e 2.35 (1H, m) 120.3 36.ge c 5.35 (1 H, br s ) 35*5e 5P 1.83 (1H, m ) 2.28 (1H, dd, 27.gf 1.52 (1H, m) 6a c 27.5f c 33.5e 35*3e J = 5.8, 3.0) 64 2.00 (1H, dd, J = 10.0, 2.0) 7a 2.91 (1H, br d, 65.7 4.93 ( 1 H, br d, 130.3 3.36 (1H, dd, 77.2 74.7 3.48 (1H, dd, J = 10.6) J = 4.5, 11.7) J = 8.5) J = 6.1, 10.3) 8 131.4d 60.4 44.6 41.4 9 C 39.8e C 38.0e C 32.2e C 32.2e C 10 C 24.2f C 22.gf C 27.5 27.4 2.83 (1H, br d, 2.93 (1H, m) 41.4 42.1 49.1 3.04 (1H, m ) 49.3 1 1 P 2.69 (1H, br d, J = 12.6) J = 12.2) 12 135.gd 135.4d 136.1d 136.1 206.4 206.7 206.2 206.2 13 14a 2.38 (1H, d, 2.36 (1H, d, 2.28 (1H, d, 2.20 (1H, d, J = 18.6) J = 18.4) J = 15.3) J = 15.8) 54.3 54.7 58.2 58.6 2.10 (1H, d, 2.02 (1H, d, 2.02 (1H, d, 140 2.12 (1H, d, J = 18.6) J = 18.3) J = 15.4) J = 15.6) 23.1 15a 1.19 ( 3 H, s) 1.23 (3 H, s ) 1.03 (3 H, s) 23.3 19.4 19.3 1.00 ( 3 H, s) 1.64 ( 3 H, d, 4.83 (1H, d, 16.1 108.0 22.9 1.57 ( 3 H, br s) 15.7 1.44 ( 3 H, br s) 16 J = 0.8) J = 1.2), 4.63 (1 H, s) 17.7 1.64 ( 3 H, br s ) 15.5 0.82 (3 H, s) 0.92 ( 3 H, s) 176 1.36 ( 3 H, s ) 9.5 9.6 147.8 18 149.0 148.2d 148.4 19 1.93 ( 3 H, s ) 1.89 (3 H, d, 25.0 1.83 ( 3 H, s) 24.4 1.91 ( 3 H, d, 24.6 24.6 J = 2.1 to 6 J = 2.0 to 6 2.93) 3.04) 20 2.26 (3 H, s) 21.8 2.22 ( 3 H, s) 21.3 2.22 (3 H, d, 22.8 2.22 ( 3 H, d, 22.8 J = 2.4 to 6 J = 2.4 to 6 2.93) 3.04) 'H NMR spectra were recorded in CDCI, a t 360 MHz. Assignments were aided by spin-decoupling and DDS experiments. J values are reported in hertz, and the chemical shifts are given in 6 units (parts per million downfield from 13CNMR spectra were recorded at 50 MHz in CDCI,. Multiplicities were obtained by single-frequency off-resoMe,Si). nance decoupling, and assignments were made on the basis of J, values when applicable and/or a comparison to models. The 6 values are in parts per million downfield from Me,&. Nonassignable proton resonances. d-f Signals within a column may be reversed.of the 11-membered ring, we employed 'H NMR experiments involving nuclear Overhauser enhancement difference spectroscopy (NOEDS).13 The results of these experiments are summarized in Figure 2. Since no enhancements of the epoxide or olefin protons were observed when the associated methyl resonances were irradiated, these groups were assigned as trans and E , respectively. The spatial arrangement of both methyls in the ll-membered ring and the bridgehead proton on the B face of the molecule were clearly established. Furthermore, the bridgehead methyl, the olefin proton, and the epoxide proton were found to be within nOe proximity on the opposite, (Y face, of the molecule. These results indicated that 1 adopts a crown conformation, in complete analogy with the corresponding 10-membered ring system (allE)-germacradiene. l4 The reactivity of (EJ)-l,5germacradienes and of the corresponding monoepoxides toward acid-catalyzed transannular cyclization has been extensively investigated. The propensity of this reaction illustrates the ease of adoption of a favorable transition state from the crown (13) Richarz, R.; Wiirthrich, K. J. Magn. Reson. 1978, 30, 147. (14) Allen, F. H.; Rogers, D. Chem. Commun. 1967, 588.
conformation.ls In order to illustrate these same features of gross structure and conformation, we investigated the capacity of 1 to undergo a similar regioselective transannular cyclization. Treatment of epoxide 1 with BF3.Et,0 in E h O yielded the isomeric tricyclic alcohols, 7 and 8, as
BF,* E t 2 0
7 R=H,R'=OH 9 R = H , R'=OAc
8 R = H , R'=OH
10 R = H , R'= OAc
11 R = R ' = 0
14 R = H , R'= R ( + ~ O M T P A 15 R:H, R':S(-)-OMTPA
the major products. Analysis of the structures of these isomers involved the interpretation of both 13C and lH NMR data, which are outlined in Table I. Both compounds were readily recognized as secondary alcohols via (15) Brown,E. D.; Sutherland, J. K. Chem. Commun. 1968,1060. For a review, see: Sutherland, J. K. Tetrahedron 1974, 30, 1651.
Look and Fenical
4132 J. Org. Chem., Vol. 47, No. 21, 1982
their facile acetylation to yield the secondary acetates, 9 and 10. The alcohols were extensively investigated by NMR methods, and their gross structures were confirmed by spin-decoupling experiments (Table I). Oxidation of alcohol 7 with pyridinium chlorochromate yielded the ketone 11. Upon oxidation, the methyl resonance a t 6 0.82 (3 H, s) in 7 shifted downfield to 6 1.11(3 H, s) in 11, thus establishing the resonance a t 6 0.82 in 7 as that due to the bridgehead methyl a t C8, adjacent to the secondary alcohol. As in the case of the epoxide 1, the conformation and relative stereochemistry of 7 was established by nuclear Overhauser enhancement difference experiments involving the irradiation of selected proton resonances. Irradiation of the C8-substituted methyl group (6 0.82,3 H, s) resulted in significant enhancement of the allylic bridgehead methine proton a t C11 but did not enhance the proton a t C3. This experiment established the cyclohexane ring as trans-fused and showed the newly formed bridgehead methyl (at C8) to be a 0substituent. These data confirmed the structures of 7 and 8, thus illustrating that 1 does undergo a regioselective cyclization analogous to that observed in the germacrene epoxide. The corresponding triene 2 was isolated from slightly less polar fractions from the initial chromatography and purified by HPLC. T h e triene analyzed for C20H,o0 by high-resolution mass spectrometry and showed infrared and UV absorptions that clearly defined the analogous a-isopropylidenylcyclopentanonegroup as in 1. The NMR features of the triene were also analogous to 1, except that the NMR bands of the epoxide were replaced in 2 by those of an additional trisubstituted olefin. Based upon this very favorable comparison, triene 2 was formulated as the C7C8 olefin precursor to 1. Treatment of 2 with 1equiv of m-chloroperbenzoic acid yielded a predictable16 mixture of two monoepoxides, 1 and 12, and diepoxide 13. The major synthetic monoepoxide obtained was identical in all respects, including its optical properties, with the natural product, 1. z m-ct-pen(ies.)
l
**
'
CH,CI,
+
+
,
0
12
13
Having established the structures and relative stereochemistries of the natural products, 1 and 2, and of the cyclization products, 7 and 8, we next defined the absolute configurations of these compounds using Mosher's 19F NMR method.17 Treatment of the tricyclic alcohol 7 with both (R)-(+)-a-methoxy-a-(trifluoromethyl)phenylacetyl (MTPA) chloride and the (5')-(-)-acid chloride in pyridine yielded the esters 14 and 15, respectively. The l?F chemical shift of the trifluoromethyl group in each derivative was measured a t 188 MHz and was found to be 5.89 ppm for 14 and 6.21 ppm (relative to external trifluoroacetic acid) for 15. Mosher's model predicts that the (SI-MTPA ester will show a CF3 resonance downfield relative to the (R)MTPA ester if the secondary alcohol possesses the S configuration. Our results indicate that the configuration a t C7 in 7 is S. Based upon the regioselective transformation of 1 to 7,the absolute configuration and structure of 1 were fully defined as 7(S),8(S)-epoxy-13-keto-l(16)Still, W.C. J. Am. Chem. SOC. 1979, 101, 2493. (17)Sullivan, G.R.; Dale, J. A.; Mosher, H. S. J.Org. Chem. 1973,38, 2143.
(S),11(R)-dolabell-3(E),12(18)-diene. Correspondingly, the triene 2 was fully characterized as 13-keto-l(S),ll(R)-dolabell-3(E),7(E),12(18)-triene. The bicyclic dolabellane skeleton has been proposed as a likely biosynthetic precursorla for tricyclic diterpenoids of the dolastanelg and/or clavularanels types, which are illustrated in the structures 7 and 8. Compounds first assigned the clavularane ring system were isolated from the octocoral Clauularia i n f l ~ t a (order '~ Stolonifera), which is related to E . calyculata. Our results clearly illustrate the chemical and structural relationship between these bicyclic and tricyclic diterpenoid ring systems. Experimental Section General. Infrared spectra were recorded on a Perkin-Elmer Model 137 spectrophotometer. Ultraviolet spectra were obtained in MeOH on a Beckman Acta XIV spectrophotometer. Proton NMR spectra were recorded in CDC13solution on a 360-MHz spectrometer constructed from an Oxford narrow-bore magnet and a Nicolet Fourier transform data system by Dr. John M. Wright of the UCSD NMR Facility; all chemical shifts are reported with respect to Me4Si(6 0). Carbon-13 NMR spectra were recorded in CDCl, solution on a Nicolet-Oxford Magnetics 50 MHz Wide-Bore spectrometer;all chemical shifts are reported with respect to Me& (6 0). Low-resolutionmass spectra were recorded at 70 eV on a Hewlett-Packard Model 5930A mass spectrometer. High-resolution mass measurements were supplied by Dr. A. Burlingame, University of California,Berkeley. Optical rotations were measured on a Perkin-Elmer Model 141 polarimeter with a 10-cm microcell. Melting points were determined on a Fisher-Johns apparatus and are reported uncorrected. All solvents used were either spectral grade or were distilled from glass prior to use. Final purifications of all metabolites and reaction products were achieved by preparative high-performance liquid chromatography on silica gel with mixtures of ethyl acetate and isooctane. Collection and Extraction. Eunicea calyculata (Ellis and Solander) (collection B-79-46) was collected by hand using SCUBA at 25 to 30 m depth in May, 1979,along the barrier reef near Carrie Bow Cay, Belize. The collection was stored in isopropyl alcohol (IPA). Upon workup, the IPA was decanted, the animal was homogenized, and the gorgonian cake was repeatedly extracted with CHC13/MeOH (21). After filtration, the combined IPA and CHCl,/MeOH extracts were evaporated under vacuum to give a residue, which was partitioned between saturated brine and CHC13. The CHC13 extract was subsequently dried over MgS04and filtered, and the filtrate was evaporated to yield 17 of a crude organic extract (from 500 g, dry weight of the gorgonian). A second collection of E. calyculata was made in Sept., 1981 in the Bahamas. The gorgonian was again found in deeper waters and collected by hand using SCUBA. The animal was stored frozen, and an organic extract was produced as described above. From both collections, 1 and 2 were eluted from a silica gel column with 25 and 15% EtOAc in isooctane, respectively, and further purified by HPLC. 7( S ),8( S )-Epoxy-13-keto- 1(S),11( R )-dolabell-3( E ) 12, (18)-diene ( I ). The epoxide 1 crystallized from diethyl ether after purification by HPLC (p-Porasil,with 30% EtOAc in isooctane). Repeated recrystallization gave 1.5 g (9% of the crude extract) of 1, mp 147-149 "C. Epoxide 1 showed [aIzoD+61" ( c 1.29, CHC13)and exhibited the following spectral features: UV ,A, (MeOH) 254 nm (e 9200); IR (CC14)2960,1700,1615,1440, 1370, 1270, 1230, 1180, 1040, 900 cm-'; HRMS: M+, m / z obsd 302.2274, C20H3002 requires 302.2246. 13-Keto-1(S),11(R)-dolabell-3(E),7(E),12(lS)-triene (2). Triene 2 was isolated as an oil. Purification by HPLC (p-Porasil (18)Braekman, J. L.; Daloze, D.; Schubert, R.; Albericci, M.; Tursch, B. Tetrahedron 1978,34, 1551. (19) Pettit, G.R.; Ode, R. H.; Herald, C. L.; Von Dreele, R. B.; Michel, C. J. Am. Chem. SOC. 1976, 98, 4677. Sun, H. H.; McConnell, 0. J.; Fenical, W. Tetrahedron 1981,37, 1237. Schmitz, F. J.; Gopichand, Y.; Michaud, D. P.; Prasad, R. S.; Remaley, S.; Hossain, M. B.; Rahman, A.; Sengupta, P. K.; van der Helm, D. Pure A p p l . Chem. 1981, 51, 853. Crews, P.; Klein, T. E.; Hogue, E. R.; Myers, B. L. J.Org. Chem. 1982, 47, 811.
New Bicyclic Diterpenoids with 10% EtOAc in isooctane) gave 2.0 g (12% of crude extract) of 2. The triene showed [alPD +31° (c 0.88, CHC13)and exhibited the following spectral features: W A- (MeOH) 256 nm (c 6900); IR (CCW 2960,1700,1615,1435,1370,1270,1200,1175,890cm-'; HRMS m/z (relative intensity) for M+, oljsd 286.2296 (ll), C.&ImOrequires 286.2297,271.2063 (Cl$InO, 5), 163.1117 (CllH16, 39), 150.1047 (C1&14, loo), 149.0966 (C1&13,39), 136.0883 (C$I12, 14), 107.0854 (CsHll, 42), 93.0707 (C7H9, 53). Reduction of Epoxide 1 To Yield 5 and 6. To a stirred solution of excess lithium in ammonia (20 mL) (dry ice-acetone bath) and anhydrous diethyl ether (10 mL) was added 85 mg (0.28 mmol) of 1 in 5 mL of diethyl ether. After 20 min, NH4Cl was slowly added, and the solution was allowed to warm to room temperature. When the ammonia had evaporated (2 h), the reaction mixture was washed with 5% HCl(2 X 20 mL), followed and fitered, and the fitrate was by 5% NaHC03, dried (MgS04), evaporated to give a mixture of two major products, 5 (25 mg, 29% from 1) and 6 (12 mg, 14% from l), which were separated by HPLC (r-Porasil, with 20% EtOAc in isooctane). These products were fully characterized. Compound 5 showed [aI2'D -126.0' (c 0.86, CHC13) and exhibited the following spectral features: IR (CC14)2960,2920,1735,1560,1455,1370,1270cm-'; MS, M+ m/z 304 for C&I32O2 (low resolution); 'H N M R (CDCI,) 6 5.44 (1 H, br d, J = 10.3 Hz), 2.74 (1 H, dd, J = 2.4 and 10.8 Hz), 2.0 (1 H, m), 1.68 (3 H, s), 1.36 (3 H, s), 1.19 (3 H, d, J = 6.9 Hz), 0.99 (3 H, s), 0.93 (3 H, d, J = 6.9 Hz). The stereochemistry of the isopropyl group in 5 was established as @ based on results from NOEDS. The doublet methyl group at 6 0.93 was shown to be within nOe proximity to the methyls at 6 0.99, 1.36, and 1.68, which are assigned to the other isopropyl methyl, epoxide methyl, and olefin methyl, respectively. Compound 6 showed [ a l a O D+10.3' (c 0.63, CHC1,) and exhibited the following spectral features: IR (CCl,) 2960,1735,1540,1465,1320,1270,1250,1030, 970 an-'; MS, M+ m/z 304 for C&sz02 (low resolution);'H N M R (CDC13)6 5.31 (1H, br d, J = 11.2 Hz), 2.81 (1 H, dd, J = 11.0 and 1.8 Hz), 1.62 (3 H, s), 1.44 (3 H, s), 1.23 (3 H, s), 1.07 (3 H, d, J = 7.0 Hz), 0.97 (3 H, d, J = 7.0 Hz). DDS and NOEDS Experiments. The decoupling difference and nOe difference spectroscopyexperimenh were performed in general as outlined by Hall and Sanders.12 However, in these experiments, since the decoupler used was not under computer control, the various irradiations were manually performed sequentially. All samples prepared for NOEDS were degassed by bubbling Ar through the solution for 45-60 min and then sealed around the cap with parafilm. Solutions were made up in CDCl, such that, after degassing and a loss of a significant volume of CDCl,, the final concentration was 0.03-0.05 M. Transannular Cyclization of 1 with BF3-Et20. Epoxide 1 (200 mg,0.66"01) was dissolved in 5 mL of anhydrousdiethyl ether, and the mixture was cooled to 0 'C. To this mixture was added 0.1 mL of BF3-etherate (distilled) with stirring. After 15 min, distilled water (10 mL) and diethyl ether were added. The ether layer was separated and washed with 5% NaHC03 (2 X 20 mL) and then dried over MgS04and fiitered, and the fitrate was evaporated to yield a mixture of products. Purification by HPLC (p-Porasil with 30% EtOAc in isooctane) gave as the major products starting material, 1 (52.0 mg, 26%), 7 (34.2 mg, 17% from l), and 8 (39.6 mg, 20% from 1). Products 7 and 8 were f d y characterized. Compound 7 showed [aImD -34' (c 0.27 CHCl,) (MeOH) and exhibited the following spectral features: UV A, 251 nm (c 9200); IR (CC4) 3550,2920,1705,1650,1540,1440,1380, 1250,1200,1030, lo00 cm-'; HRMS, M+ m/z (relative intensity) obsd 302.2253 (31), CzoHm02requires 302.2246,269.1897(M+HZO, M+ - CH3, C1&%0,4), 107.0862 (C&11,37), 105.0704 (CsH9, 29), 95.0865 (C7H11, 35), 91.0545 (C7H7, 41), 83.0496 (C5H7, 100). Compound 8 crystallized from EtOAc, mp 132-134 "C and gave [ a I m D -51' (c 0.74, CHC13) and exhibited the following spectral characteristics: U V A,- (MeOH) 252 nm (E 6100); IR (CClJ 3550, 2920,1705,1615,1440,1375,1245,1200,1155,1080cm-'; HRMS, M+ m/z (relative intensity) obsd 302.2234 (loo), C&,02 requires 302.2246,284.2123 (M+- H20, C&ImO, 7), 269.1912 (M+- H20, CH3, C1&%0, 9), 204.1499 (C14Hm0,39), 149.0962 (C1&13O, 35), 123.0811 (CBHiiO, 34). Acetylation of 7 and 8 To Give 9 and 10. To 5 mg of compound, dissolved in 2 mL of dry pyridine, excess acetic anhydride was added with stirring. The reaction mixture was stirred at room
J. Org. Chem., Vol. 47, No. 21, 1982 4133 temperature overnight and then quenched with ice and extracted with diethyl ether (2 x 20 mL). The combined ether layers were washed with 5% HCl(2 X 20 mL), water, and 5% NaHC03 (20 mL), dried over MgS04, and filtered,and the filtrate was evaporated to yield, quantitatively, the crude ester. Both 9 and 1 0 were purified by HPLC (p-Porasil with 20% EtOAc in isooctane). Acetate 9 showed [a]"OD -50' (c 0.3, CHC1,) and exhibited the (MeOH) 252 (c 11700); IR following spectral features: UV A, (CHC13)2950,1735,1705,1610 cm-'; MS: M+ m/z 344 for CZ2H3203 (low resolution); 'H NMR (CDC1,) 6 4.86 (1H, s), 4.65 (1 H, br s), 4.63 (1H, dd, J = 11.8 and 4.5 Hz), 2.91 (1 H, br m), 2.22 (3 H, d, J = 2.4 Hz to 6 2.91), 2.08 (3 H, s), 1.89 (3 H, d, J = 1.9 Hz to 6 2.91), 1.01 (3 H, s),0.91 (3 H, s). Acetate 1 0 showed [aI2'D -33' (c 0.1, CHC13) and exhibited the following spectral (MeOH) 251 nm (c 7700); IR (CHCl,) 2950, features: UV A, 1735,1705,1605 cm-'; MS, M+ m/z 344 for C22H3203 (low resolution); 'H NMR (CDC1,) 6 5.35 (1H, br s), 4.73 (1 H, dd, J = 10.4 and 6.2 Hz), 3.02 (1H, br m), 2.22 (3 H, d, J = 2.0 Hz to 6 3.02), 2.07 (3 H, s), 1.90 (3 H, d, J = 1.5 Hz to 6 3.02), 1.64 (3 H, s), 1.01 (3 H, s), 0.98 (3 H, s). Oxidation of 7 To Yield Ketone 11. One equivalent of pyridinium chlorochromatewas added to a stirred solution of 7 (10 mg)in CHzC12(2 mL) at room temperature. After 2 h, the reaction mixture was diluted with CH2C12(15 mL) and filtered through a small silica gel column to yield ketone 11 as an oil in quantitative yield. Ketone 1 1 showed [aI2OD +23.1' (c 0.51, CHCl,) and ex(MeOH)252 nm hibited the following spectral features: UV ,A, (c 7000); IR (CHC13) 2920, 1705, 1620; MS, M+ m/z 300 for C2&m02(low resolution); 'H NMR (CDCl,) 6 5.05 (1H, s), 4.88 (1H, s), 2.91 (1H, br m), 2.22 (3 H, d, J = 1.8 Hz to 6 2.91), 1.90 (3 H, d, J = 1.5 Hz to 6 2.911, 1.11 (3 H, s), 0.99 (3 H, s). Epoxidation of 2. To a stirred solution of 2 (61.2 mg, 0.21 mmol) in 10 mL of CHzC12buffered with anhydrous Na2HP04 (buffering was necessary in order to prevent transannular cyclizations)was added 1.2 equiv of m-chloroperbenzoicacid. After stirrii for 2 h at room temperature, the CH2C12layer was washed with 10% sodium sulfite (2 x 20 mL), 5% NaHCO, (2 x 20 mL), and, finally, with brine. After drying over MgS04and filtration and evaporation of the CH2C12,the resulting oil was chromatographed by HPLC (45% EtOAc in isooctane). The reaction mixture gave two monoepoxides, 1 (29 mg, 45% yield from 2) and 12 (21 mg, 32% yield from 2), and diepoxide 13 (5 mg, 7% yield from 2). The major product of the reaction was identical with the natural product, 1 ([aIa0D +59' (c 0.48, CHCl,)). Monoepoxide 1 2 showed [aIa0D -54.1' (c 0.93, CHCld and exhibited the following (MeOH) 255 nm (t 5700);IR (CHCI,) spectral features: UV A, 2950,1700,1610,1460cm-'; HRMS, M+ m/z (relative intensity) obsd 302.2233 (3), C&Im02 requires 302.2246, 163.1117 (CllC150, 34), 149.0964 (c1&130,93), 135.0809 (c$I~,,o, 72), 95.0860 (C7H11, 70), 93.0705 (C7HB,94); 'H NMR (CDCl,) 6 5.12 (1H, br d, J = 11.1Hz), 2.99 (1H, dd, J = 11.1and 2.9 Hz), 2.94 (1H, br d, J = 11.9 Hz), 2.41 (1H, d, J = 18.2 Hz), 2.20 (3 H, s), 2.11 (1H, d, J = 18.4 Hz), 1.84 (3 H, s), 1.73 (3 H, s), 1.41 (3 H, s), 1.14 (3 -26.5' (c 0.83, CHC1,) and H, s). Diepoxide 1 3 showed [aI2O~ (MeOH) 253 exhibited the following spectral features: UV A, nm (e 6100); IR (CHCld 2920,1705,1610,1450,1380cm-'; HRMS, M+ m/z (relativeintensity) obsd 318.2210 (45),C&Im03requires 318.2195,149.0968 (C1&13O, 100), 136.0892(C&120,71); 'H NMR (CDCl,) 6 3.07 (1H, dd, J = 13.9 and 2.7 Hz), 2.97 (1 H, br d, J = 7.7 Hz), 2.82 (1H, br d, J = 12.5 Hz), 2.45 (1H, d, J = 18.3 Hz), 2.24 (3 H, s), 1.95 (3 H, s), 1.43 (3 H, s), 1.34 (3 H, s), 1.22 (3 H, 4. Synthesis of a-Methoxy-a-(trifluoromethy1)phenylacetyl Chlorides (MTPA-Cl). (+)-a-Methoxy-a-(trifluormethy1)phenylacetic acid (1.0 g, Aldrich Chemical Co.), thionyl chloride (2 mL, distilled), and sodium chloride (10 mg) were combined and refluxed for 60 h. Excess thionyl chloride was removed by vacuum distillation (47 "C, 85 mm), and the remaining residue was distilled to give the acid chloride (83-85 'C, 8 mm). The (-)-MTPA-C1was synthesized in the same manner. Synthesis of Esters 14 and 15. To 8.3 mg of 7 in 2 mL of dry pyridine was added 1equiv of (+)-MTPA-Cl in CC14(1mL). After stirring overnight at room temperature, ice was added and the mixture was extracted with diethyl ether (2 X 20 mL). The ether layer was washed with 5% HC1 (2 X 20 mL) and 5% NaHCO,, dried over MgSO,, and filtered, and the filtrate was
J. Org. Chem. 1982,47,4134-4137
4134
evaporated to give 14 as an oil in quantitative yield. Compound 14 was purified by HPLC (r-Porasil, 25% EtOAc in isooctane). The 19F NMR chemical shift for the CF3 group in ester 14, measured in CDC13solution at 188 MHz, was determined to be 5.89 ppm (downfieldfrom extend trifluoroaceticacid in CDC13). Ester 15 was synthesized by reaction of 7 (10 mg) with (-)MTPA-C1 and purified as described above. The CF, group for ester 15 gave a 19FNMR chemical shift of 6.21 ppm.
Bio-organic, Biomedical Mass Spectrometry Resource Center (A. L. Burlingame, Director), University of California, Berkeley, supported by NIH Grant RR00719 from the ~ i ~of Research i ~ i R ~ ~ ~ This research ~ is a~ result of financial support provided by NoAA, Office of sea Grant, Department Of Commerce, under Grant 04-'M-01-189 (project R/MP-22).
Acknowledgment. This report is contribution No. 117 of the IMSWE Program, Reef and Mangrove Study in Belize, sponsored by the Smithsonian Institution, and
Registry No. 1, 82798-97-8; 2, 82798-98-9; 5, 82798-99-0; 6, 82838286; 7,82799-00-6;8,82799-01-7;9,82799-02-8; 10,82799-03-9; 11, 82799-04-0; 12, 82799-05-1; 13, 82799-06-2; 14, 82799-07-3; 15, 82838-29-7; (+)-a-methoxy-a-(trifluoromethy1)phenylaceticacid, 20445-31-2;(+)-a-methoxyw(trifluoromethy1)phenylacetylchloride, 20445-33-4; (-)-a-methoxy-a-(trifluoromethyl)phenylaceticacid, 17257-71-5;(-)-a-methoxy-w(trifluoromethy1)phenylacetyl chloride, 39637-99-5.
by the 'Orp* We thank Dr' Klaus Ruetzler for the kind invitation t0 participate. We thank Dr. Katy Muzik for assistance in identifying gargonian corals. Mass spectra were kindly provided by the
supported' in
Spectroscopic, Optical, and Crystallographic Properties of ( S)-( )-cis-6'-Bromo-N-formylnorreticuline
+
Peter Buchs,l Kenner C. Rice, and Arnold Brossi* Section on Medicinal Chemistry, Laboratory of Chemistry, National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20205
J. V. Silverton Laboratory o f Chemistry, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20205
Rudolph Potenzone, Jr. Environmental Protection Agency (MZDSD), Washington, DC 20460 Received March 9, 1982
A rotamer mixture of (8)-N-formyl-N-norreticuline (I) was brominated regioselectively in the 6'-position to afford the corresponding mixture of 6'-bromo rotamers (2) from which the thermodynamically less stable cis rotamer could easily be isolated by direct crystallization. This rotamer was stable in crystalline condition but equilibrated to a 1:2 cis-trans mixture on standing in solution. This equilibration was easily followed by observation of changes in optical rotation and the ratio of N-formyl protons in the NMR spectrum. Assignment of the cis structure to the rotamer isolated by crystallization was confirmed by a single-crystalX-ray analysis, which also confirmed the absolute configuration of this rotamer made independently from established stereochemical relationships. The crystals of 2 have space group P2, and cell dimensions a = 6.616 (1) A,b = 10.985 (1)A,c = 13.084 (2) A and /3 = 95.61 (1)'. The structure was refined to an R factor of 4.3%. The presence of rotamer pairs was repeatedly observed in the-series of 1-benzyl-substituted N-formyl-1,2,3,4tetrahydroisoquinolines (TIQ) by routine TLC and 'H NMR analy~is.~" The latter method can be used to quantitate the individual amounts of cis and trans rotamers (cis = C1-N bond cis to C=O of NCH0I5 by the different chemical shifts of the N-formyl proton. Recently Olieman and van Koningsfeld6 presented the first X-ray crystallographic study of the structure of an N-formyl-l(1) Visiting fellow from Swiss Federal School of Technology, _.Zurich, Switzerland. (2) Rice, K. C.; Brossi, A. J. Org. Chem. 1980, 45, 592. (3) Rice, K. C.: Ripka, W. C.; Reden, J.: Brossi, A. J.Ora. Chem. 1980, 45,601. (4)Brossi, A. Proceedings of 4th Asian Symposium on Medicinal Plants and Spices, Bangkok, Thailand, September 1980, p 261. (5) Szhtay, C.; BlaskB, G.; Bbczai-Beke, M.; Ddrnyei, G. Heteroey-
des 1980, 14, 1127.
(6) van Koningsfeld, H.;Olieman, C. Cryst. S t r u t . Common. 1981, IO,
541.
Table I cis-trans [ a 1"D ratioa aobsd deg 81.5 83:17 0.142 1 5 80.9 54:46 0.686 2 30 77.5 41 :59 0.6 57 3 60 33:67 0.636 74.9 4 180 a Determined by integration of the formyl proton N M R resonance in 0.85% solution in Me,SO. In 0.85% solution in Me,SO. expt
time, min
9
benzyl-l,2,3,4,5,8-hexahydroisoquinoline, a close relative of this class of compounds. We now supplement this information by describing the physical and crystallographic properties and particularly the optical behavior of another member of the class of TIQ.4 Bromination of (S)-(+)-N-formyl-N-norreticuline (l), consisting of a rotamer mixture,2afforded a cis-trans rotamer mixture of the corresponding (S)-(+)-g'-bromo-N-
This article not subject to US.Copyright. Published 1982 by the American Chemical Society
~
